This invention relates generally to absorption refrigeration, and more specifically to absorption refrigeration systems that utilize both a high temperature energy source and a low temperature energy source to generate a concentrated absorbent solution.
Generally, an absorption refrigeration system includes an evaporator, an absorber, a generator, and a condenser and uses an absorbent fluid such as lithium bromide and a refrigerant fluid such as water. The absorbent fluid is located in the absorber, the refrigerant fluid is located in the evaporator, and the absorber and evaporator are connected together so that refrigerant vapor can pass from the evaporator to the absorber. The absorbent fluid has an affinity for refrigerant vapor and absorbs refrigerant vapor that has passed from the evaporator to the absorber. This produces a pressure drop in the evaporator allowing more refrigerant to evaporate. A fluid such as water is circulated through a heat exchanging coil, referred to as the chilled water coil, which is positioned in the evaporator and connected as part of a closed loop fluid circuit to a refrigeration load. As refrigerant evaporates in the evaporator, it absorbs heat from the water passing through the chilled water coil. The water then absorbs heat from the refrigeration load such as an enclosure which is to be cooled.
As the absorbent fluid absorbs refrigerant vapor, it becomes diluted by the refrigerant and its affinity for refrigerant vapor decreases. In order to separate the absorbed refrigerant from the absorbing fluid and thereby obtain a concentrated absorbent fluid, the solution of refrigerant fluid and absorbent fluid is pumped from the absorber to the generator. A heat exchanging coil is positioned in the generator and a heated fluid such as steam from a source of heat such as an oil-fired steam generator is circulated through the coil. Heat is transferred from the coil to the solution of refrigerant and absorbent fluid and this heat vaporizes, or "boils off," refrigerant that had been absorbed by the absorbent fluid. The absorbent fluid is, thus, concentrated and the concentrated absorbent fluid is then returned to the absorber. The refrigerant vapor that was evaporated from the absorbent fluid in the generator passes to the condenser. In the condenser, it is condensed by a cooling fluid being passed through a heat exchanging coil that is positioned in the condenser. The condensed refrigerant then flows back to the evaporator where a new cycle can begin. Customarily, a heat exchanging coil is positioned in the absorber to cool the absorbent fluid as that fluid enters the absorber, and this heat exchanger is connected in series to the heat exchanger in the condenser. In this manner, only one source of relatively cool water such as a conventional water cooling tower is needed for the entire refrigeration system.
The heat needed to vaporize refrigerant from the solution of refrigerant and absorbent fluid; that is, to generate a concentrated absorbent fluid, can be provided by any suitable energy source such as steam, an electric resistance heater, or an oil-fired furnace. In absorption refrigeration systems that use these traditional sources of heat, the fluid that is passed through the heat exchanger in the generator is raised to about 250.degree. F. before entering the heat exchanger. Recently, much attention has been directed toward using low temperature energy sources to generate a concentrated absorbent fluid. For example, consideration has been given to solar energy, geothermal energy, and the waste heat produced by many manufacturing processes. These low energy sources usually cannot provide a working fluid that has a temperature greater than 200.degree. F., and often the fluid has a temperature of only 170.degree. F. or 180.degree. F. Also, these sources, particularly solar energy, are not as reliable as conventional energy sources. Many of the problems caused by the relatively low temperature and unreliability of these low temperature energy sources can be mitigated by using, in one way or another, a conventional high temperature energy source to augment or supplement the low temperature energy source. However, when a high temperature energy source is used in such a situation, the high energy source tends to contribute most of the required energy input while the low energy system, in effect, becomes a back up source. Since the low energy source is usually much less costly than the high energy source, in order to insure an economical operation of the absorption refrigeration system, the low energy source must contribute as much energy as possible both before and while the high energy source is asked to contribute energy to the system.